The present invention relates to structural engineering adhesives for bonding metal and fiber-reinforced composite parts (e.g. sheet molding compounds (SMC), fiberglass reinforced polyesters (FRP), structural reaction injected molded (SRIM), resin transfer moldings (RTM), and the like) to a variety of similar and dissimilar substrates, which find use in the manufacture of cars, trucks, boats, and a host of other products.
Sheet molding compound (SMC), for example, is defined (ASTM) as a molding compound in integral sheet form comprising a thermosetting resin, fibrous reinforcement, and additives required for processing or product performance, e.g., resin, catalyst, thickener, mold release agent, particulate filler, pigment, shrink control agent, etc. These materials and others generally are known as fiber-reinforced composites, reinforced composites, or simply composites. Metal may include, but not be limited to, hot dipped galvanized steel, electrogalvanized steel, e-coat steel, cold rolled steel, bare aluminum, anodized aluminum, etched aluminum, magnesium, etc.
One typical class of structural adhesives useful in adhering composite parts to the same and to different substrates is two-part polyurethane adhesives. Combining a prepolymer and a curative just before use makes these materials useful as adhesives. The ratio in which these materials are combined will vary depending upon the functionality of the prepolymer and the curative. Accurate combination of the materials requires a certain skill level of the worker and, unfortunately, there is substantial waste of adhesive during the mixing process even when using automatic pumping equipment. In addition, polyurethane adhesives generally have poor thermal properties and are not conducive to applications requiring high temperature ovens.
Another common class of structural adhesives useful in adhering metal parts to the same and to different substrates (e.g. composites) is epoxy adhesives. Epoxy adhesive compositions most often contain a polyfunctional epoxy resin and are cured by addition of a curative, which typically is provided in a separate package. The rate of cure and product characteristics are influenced by the choice of curing agent, which itself is influenced by the make-up of the adhesive composition, as dictated by the final properties desired by the user.
Structural adhesives are used by application to the surface of a part made of, e.g. metal, and positioning a surface of second part (of the same or different material) over the adhesive covered metal covered surface. Since the parts often have uneven surfaces, it is desirable that the adhesive possess the ability to fill the resulting voids of varying depth. It is important that the adhesive remain uncured and fluid for sufficient time to permit placing of the second substrate into contact with the adhesive. An adhesive, which hardens too quickly, does not permit flexibility in the assembly line process. Thus, the length of time the adhesive is fluid is measured and is referred to as xe2x80x9copen timexe2x80x9d. The adhesive may be cured by placing the adhered parts in an oven maintained at, e.g., 70xc2x0-190xc2x0 C. for, e.g., 5 minutes or less to cure or harden the adhesive, or the adhesive may be cured by letting it stand at room temperature for one to several days, e.g., 3 days.
Representative epoxy structural adhesive compositions can be found in, for example, U.S. Pat. Nos. 5,385,990, 4,921,912, 4,661,539, 4,740,539, and 4,707,518, the disclosures of which are expressly incorporated herein by reference. Various combinations of epoxy resins, rubber modifiers, amine curing agents, amide curing agents, Lewis acids, mercaptan curing agents, etc. have been proposed for formulating high strength adhesive compositions. A major deficiency in these adhesives is that they often suffer from poor and/or limited adhesion, especially to SMC, SRIM, vinyl ester SMC, and other substrates. These same adhesives also require post-baking in order to obtain full cure and properties.
Broadly, the present invention embodies an adhesive composition, which comprises an epoxy resin, a coupling agent, filler, and an effective amount of an amine-curing agent or curative for said epoxy resin. Advantageously, tri-functional and/or tetrafunctional epoxy resins and/or acrylate monomers will be incorporated into the adhesive composition in order to reduce open time and enhance substrate adhesion. Advantageously, a mixture of amines will be used in the curative including aliphatic amines, which have low viscosities and efficiently wet the substrate for enhancing adhesion; polyamines, which can be used to manipulate open time and allow for improved ratio tolerance of the adhesive system; and amine-terminated rubbers (ATBN), which can improve impact resistance and the toughness of the cured adhesive. Preferred coupling agents are silanes. In addition, dicyandiamide may be incorporated in the amine side to react with any residual epoxy resin that may be present.
Advantages of the present invention include an adhesive composition, which does not need any post-baking to obtain full cure. Another advantage is excellent adhesion that the inventive adhesive composition retains without post-baking. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.
Conventional two-component epoxy adhesives are extensively used in the automotive industry to bond SMC, SRIM (structural reaction injected molding), and other substrates. However, many of these epoxy systems, particularly polyamide-based systems, have undesirably long open times and require post-baking in order to achieve full cure of the adhesive composition. The inventive adhesive composition overcomes such problems by incorporation of a coupling agent with the epoxy resin and by using aliphatic amines as part of the curative.
Referring initially to the epoxy resin, a variety of monomeric and polymeric compounds or mixtures of compounds having an epoxy equivalency equal to or greater than 1 (i.e., wherein the average number of epoxy groups per molecule is 1 or more) can be used in formulating the inventive adhesives. Epoxy compounds are well-known as the art cited above details and which is expressly incorporated herein by reference. Useful epoxy compounds include, for example, polyglycidyl ethers of polyhydric polyols, such as ethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,5-pentanediol, 1,2,6-hexanetriol, glycerol, and 2,2-bis(4-hydroxy cyclohexyl)propane; polyglycidyl ethers of aliphatic and aromatic polycarboxylic acids, such as, for example, oxalic acid, succinic acid, glutaric acid, terephthalic acid, 2,6-napthalene dicarboxylic acid, and dimerized linoleic acid; polyglycidyl ethers of polyphenols, such as, for example, bis-phenol A, bis-phenol F, 1,1-bis(4-hydroxyphenyl)ethane, 1,1-bis(4-hydroxyphenyl)isobutane, and 1,5-dihydroxy napthalene; modified epoxy resins with acrylate or urethane moieties; glycidlyamine epoxy resins; and novolak resins; and the like and mixtures thereof.
The foregoing epoxy resins may be augmented with modified epoxy resins in the form of epoxy-rubber adducts. Such adducts are well known and include epoxy compounds reacted with liquid or solid butadiene-(meth)acrylonitrile copolymers having at least two groups which are reactive with epoxy groups, including, for example, carboxyl, hydroxyl, mercapto, and amino. Such functional elastomeric copolymers having functional groups are well-known articles of commerce and need not be discussed in greater detail herein. It should be recognized additionally, that such rubber compounds also can be added to the curative pack of the two-pack structural adhesive of the present invention. Thus, the rubber modifier can be used neat or adducted with, for example, an epoxy; and included in one or more of the resin pack or the curative pack of the inventive adhesive composition. A preferred rubber is an amine terminated butadieneacrylonitrile rubber, which may be present in an amount ranging up to about 65 wt-% and advantageously between about 30 and 50 wt-%.
Referring now to adhesion promoters, such promoters include the reaction product of an omega-aminoalkyl trialkoxy silane with a glycidyl ether or polyglycidyl ether. Typical trialkoxy silane linkages include xe2x80x94Si(OCH3)3 and xe2x80x94Si(OCH2CH3)3, which are capable of hydrolyzing to Si(OH)3. Suitable epoxy functional silane compounds include, for example, gamma-glycidoxypropyltrimethoxysilane and beta-(3,4-epoxycyclohexyl)ethyltrimethoxy silane. In addition, organo-silanes containing moieties, such as, for example, ester, vinyl, methacryloxy, sulfur, amino, ureido, isocyanurate, and isocyanato groups may be used. The silane ingredient can range up to about 10 wt-% and advantageously ranges from about 0.25 to 2 wt-% in the adhesive composition.
Acrylate monomers may be incorporated in the aforementioned epoxy resins to further adjust open time and improve adhesion, in particular, to metal substrates. Acrylate monomers may be used solely or as a mixture of two or more monomers. Suitable acrylate monomers include, for example, monofunctional, difunctional, tri-functional, and tetrafunctional acrylates. A representative listing of these monomers includes alkyl acrylates, hydroxyalkyl acrylates, alkoxyalkyl acrylates, acrylated epoxy resins, cyanoalkly acrylates, alkyl methacrylates, hydroxyalkyl methacrylates, alkoxyalkyl methacrylates, cyanoalkyl methacrylates, N-alkoxymethacrylamides, N-alkoxymethylmethacrylamides, and difunctional monomer acrylates. Other acrylates which can be used include trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, zinc diacrylate, 2-ethylhexyl methacrylate, pentaerythritol triacrylate and pentaerythritol tetraacrylate. The foregoing list is merely illustrative and not limitative of the present invention.
Referring now to the amine-based curing agents, such curing agents include aliphatic amines, polyamines, polyamidoamines, alicyclic polyamines, tertiary amines, and various mixtures thereof. Suitable aliphatic amines and polyamines include, but are not limited to, ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, 2-methyl-1,5-pentanediamine, pentaethylenehexamine, hexamethylenediamine, trimethyl-1,6-hexanediamine, polyetherdiamine, diethylaminopropylamine, oleylamine, isophorone diamine, diethanolamine, triethanolamine, tris(dimethyl)aminoethylphenol, dimethylaminomethylphenol, dicyandiamide, diaminodiphenylsulfone, bis(aminopropyl)piperazine, and N-aminoethylpiperazine. Suitable aliphatic polyamines include resins, which are modified, for example, by condensation with tall oil fatty acids. Furthermore, Mannich bases and aromatic polyamines, such as, for example, xylenediamine, may be used as amine hardeners. The aliphatic amine should be present in an amount of between about 1 and 65 wt-% and advantageously between about 1 and 35 wt-%. The polyamine should be present in an amount of between about 1 and 65 wt-% and advantageously between about 1 and 35 wt-%.
In order to obtain high flexibility, toughness and improved water stability, amidoamines or polyamides may be used. Amidoamines can contain flexible groups, in particular the dimerized linoleic acid backbone. These materials may be obtained from commercials sources, i.e., Versamid 140 (the reaction product of dimerized linoleic acid with aliphatic primary amines, Henkel). Furthermore, to enhance the rates of curing, flexibility and toughness, polyphenolics such as bisphenol-A can also be included in the hardener component. The active hydrogen equivalent weight of the hardener can by varied by adding different levels of poly(alkylene ether) diamine. This also helps to improve the flexibility and adhesion of the adhesive. The amount (ratio) of amidoamine:bisphenol-A may be in the range of 30 to 90:8 to 35:2 to 35 weight percent.
Optional ingredients in the adhesive composition include, for example particulate and reinforcing fillers and thixotropic agents, tinctorial pigments, opacifying pigments (e.g., TiO2), and like conventional additives. Fillers are utilized in the adhesive to help maintain viscosity, improve sag resistance, and provide reinforcement to the final cured material, as well as reduce the final cost of the product. Useful fillers include, for example, Kevlar(copyright), kaolin, talc, mica, clay, calcium carbonate, any of the alkaline earth inorganic salts, metals such as powdered aluminum or iron, metal oxides such as ferric oxide or aluminum oxide, silica, ceramic beads such as those available under the trademark Zeeospheres from Zeelan Industries, Inc., or any other filler (and mixtures thereof) well-known to those skilled in the art of formulating adhesives.
The adhesive of the present invention is particularly well adapted for use on a variety of fiber-reinforced composites, including, for example, sheet molding compound (SMC), structural reaction injected molded (SRIM), vinyl ester SMC, and E-coat metal substrates. Among the fiberglass reinforced polyester substrates useful in the practice of this invention are those provided by Ashland Specialty Chemical, Dublin, Ohio (Phase xcex2, Phase xcex4, Phase xcex5), GenCorp, Marion, Ind. (GC-7113, GC-8002 and GC-7101 substrates), Rockwell International Corporation, Centralia, Ill. (RW 9468 Substrate), Budd Company, Madison Heights, Mich. (DSM 950 and DSM 951 Substrate), and Eagle Picher Plastics, Grabill, Ind. (EP SLI-213 Substrate). The SRIM substrates useful in the practice include those provided by Bayer, Pittsburgh, Pa. (Baydur 425 HD-SRIM). Typical vinyl ester SMC substrates are manufactured by Dow, Midland, Mich. (Derakane 790). Car and truck body parts made of sheet molding compound (SMC) also are adhered using structural urethane adhesives and can now be adhered using the two-part epoxy adhesive of this invention.
The inventive adhesive is adaptable for use on a variety of other plastics such as reaction injection molding (RIM) polyurethanes, acrylonitrile-butadiene-styrene (ABS) terpolymers, styrene acrylonitrile copolymers (SAN), nylon, thermoplastic polyolefins (TPO), and thermoplastic alloys such as, for example, polycarbonate-polyester blends and polycarbonate-ABS blends. Among the useful fibers used in reinforcing the substrates are fiberglass, graphite, and polymeric fibers, e.g., polyamide fiber. The inventive adhesive further can be used to adhere SMC to metal, optionally primed, for example, with electrodeposited (ELPO) primers.
The adhesive composition is formulated by simple blending, often under high shear conditions, of the ingredients. For SMC uses, the adhesive composition preferably is applied robotically by extrusion through a follower plate, though it may be applied by conventional roller coating, both direct and indirect, spray application, dip application, or any application technique that is necessary, desirable, or convenient. No priming of the composite or metal substrate is required when using the inventive adhesive. The parts then are joined under pressure at ambient temperature or, optionally, elevated temperatures (i.e., greater than 82xc2x0 C.) to facilitate cure.
While the invention has been described and illustrated in connection with certain preferred embodiments thereof, it will be apparent to those skilled in the art that the invention is not limited thereto. Accordingly, it is intended that the appended claims cover all modifications, which are within the spirit and scope of this invention. All references cited herein are expressly incorporated herein by reference.
The following examples show how the invention has been practiced, but should not be construed as limiting. In this application, all percentages and proportions are by weight and all units are in the metric system, unless otherwise expressly indicated.